The importance of the story is the epiphany that the boy has after actually visiting Araby. He is young and has great aspirations about life and love. When he comes to realize how cruel life can really be, his epiphany is clear. Not everything we plan for works out for us. If he had been older, it wouldn't have been so believable. But this is evidently his first bout with "love." This boy is important because of his age and his innocent crush that he has on his friend's sister.
Another important point has to do with the theme of religion. This boy seems to think that he is a religious hero to the girl he has a crush on. All of the religious undertones show how his expectations of his life intertwined with his beliefs is challenged at the end. Because Joyce used this boy, the reader is left at the end trying to decide how his views will change once he gets back home and attends church and other religious gatherings, or if he even will any more. Will the boy end up like Joyce did with his Catholicism?
In a way, we can see this as a story of interior vs. exterior, inside vs. outside. The boy's emotional experience is set against the indifference of the exterior world. When the boy attempts to merge the two spaces by epitomizing his affections for Mangan's sister with a trinket from the bazaar, his internal reality comes into conflict with the external reality.
This all supports the idea of the necessity for a first person narrative perspective, I believe.
"Araby" is a coming-of-age story, meaning that through what he experiences, the boy matures a little bit. Since the story is concerned with his feelings and thoughts and growth, then it is only natural that he be the person telling the story. It would not mean the same thing if an outsider had to tell us what the boy was thinking or what kind of lesson he learned.
Well, for one, if it were a girl narrator, she wouldn't be obessessed with Mangan's sister.
Since Araby is the account of adolescent love, rather obsessive and inward-moving, the story is rightly told by the boy-hero himself in the first person narrative mode. The boy's secretly growing passion for Mangan's sister and the sort of pilgrimage that the boy undertakes could not have been told by anybody else. Araby is a cofessional story which records in a very emotive language the growth of the boy's mind within the limits of daily existence, and beyond those limits to a land of dreams. The boy-hero cum narrator describes in his own person, the confusing journey away from the drab Dublin life. The story ends on a note of disillusionment and epiphany. Such a story, much 'more than a story', is rightly rendered in the boy's very own first person mode.